The present disclosure relates to hydraulic control devices for automatic transmissions that include a plurality of engagement elements and that are mounted on, e.g., vehicles, and more particularly to hydraulic control devices for automatic transmissions which have a limp home function for off-failure of linear solenoid valves that can supply an oil pressure to hydraulic servos of a plurality of engagement elements.
Conventionally, stepped automatic transmissions that are mounted on, e.g., vehicles can implement multi-speed shifting by controlling the engagement states of a plurality of engagement elements (clutches and brakes) by a hydraulic control device so as to form a transmission path in a speed change mechanism at each shift speed. Among such hydraulic control devices, those hydraulic control devices are widely used which include linear solenoid valves that can supply an oil pressure to hydraulic servos for engaging and disengaging the engagement elements, and which have a limp home function for all-off failure, or failure in which all of solenoid valves such as the plurality of linear solenoid valves are turned off (a function to establish a shift speed even in case of all-off failure such that the vehicle can be driven). Hydraulic control devices for automatic transmissions are under development which, as such a limp home function, reversely inputs a limp home pressure to drain ports of linear solenoid valves, so that those engagement elements corresponding to the shift speed in a limp home mode can be engaged (see Japanese Patent Application Publication No. 2005-344741).
Such a hydraulic control device includes a failsafe valve that outputs a range pressure as the limp home pressure in case of all-off failure of the solenoid valves. In this hydraulic control device, when the solenoid valves are operating normally without causing all-off failure, oil discharged from the drain port of the linear solenoid valve flows through the failsafe valve and is then discharged through a drain check valve.